Parallel-pumped nonlinear loss effects for polycrystalline yttrium iron garnet in the high-power microwave environment

In this study, the parallel-pumped nonlinear (NL) loss effects of polycrystalline Yttrium Iron Garnet (YIG) material caused by the high-power microwave (HPM) were explored for the first time. In terms of the coupling process of spin waves and electromagnetic waves, the improved ferrite NL theory encompassing the input microwave waveform parameters (pulse duration and power) was developed. The theoretical and experimental results indicated that the critical threshold power of the NL effect is approximately inversely proportional to incident microwave pulse with a duration below 80 ns, demonstrating the features of the energy threshold. Furthermore, the theoretical calculation and the high-power measurements show that the polycrystalline YIG sample exhibits a relatively stable microwave power loss trend in the HPM environment with input power of 160 W–400 W. The influence of the external magnetic bias field on the YIG nonlinear effects was also carried out for comparative analysis. This research not only expanded the NL loss theory of ferrite in the nanosecond-level microwave environment but also offered a potential limiting protection application against HPM electromagnetic environment threats.     

AN INVESTIGATION OF THE HEATING OF WOOD IN AN INDUSTRIAL MICROWAVE APPLICATOR: THEORY AND PRACTICE

In this work a comprehensive set of experimental results are used as an excellent means to understand the coupling that exists between the material and die electromagnetic fields in a specific industrial microwave applicator. The analysis of the infrared images allows an accurate map of the power and temperature distributions within the wood sample to be determined. This map, together with the simulation results of a previously developed computational electromagnetic model, can provide a detailed understanding of the design features of the microwave applicator. In particular, it is possible to locate the occurrence of localised hot spots and to examine the uniformity of the heat distribution throughout the sample. The simulation results provide the evolution of the electromagnetic fields inside the entire applicator and the sample. The coupling of theory and practice is the best way to proceed in optimising the design and for proposing new applicator geometry that can heat the material more effectively.     

AN INVESTIGATION OF THE HEATING OF WOOD IN AN INDUSTRIAL MICROWAVE APPLICATOR: THEORY AND PRACTICE

ABSTRACT

In this work a comprehensive set of experimental results are used as an excellent means to understand the coupling that exists between the material and die electromagnetic fields in a specific industrial microwave applicator. The analysis of the infrared images allows an accurate map of the power and temperature distributions within the wood sample to be determined. This map, together with the simulation results of a previously developed computational electromagnetic model, can provide a detailed understanding of the design features of the microwave applicator. In particular, it is possible to locate the occurrence of localised hot spots and to examine the uniformity of the heat distribution throughout the sample. The simulation results provide the evolution of the electromagnetic fields inside the entire applicator and the sample. The coupling of theory and practice is the best way to proceed in optimising the design and for proposing new applicator geometry that can heat the material more effectively.     

Simulation of Microwave Sintering of Ceramic Bodies with Complex Geometry

Microwave sintering, an emerging technology in which the energy is applied directly to the material, enabling rapid sintering, shows potential for the synthesis of advanced ceramic materials with superior properties. The process is complex, combining the propagation and absorption of electromagnetic waves in the ceramic material, heat transport within the geometric body, and densification. The densification changes both macroscopic shape and microstructural morphology. A dynamic balance between the rate of electromagnetic energy absorbed within the bulk of the sample and the rate of energy loss from its surface generally results in temperature gradients. These temperature gradients may be especially important during the microwave sintering of bodies with a complex geometry, because neither the diffusion distance nor the electromagnetic penetration depth scale with sample dimensions. The gradients generated in a ZnO green body of a complex geometry were studied theoretically using various microwave-sintering approaches, and it was found that (1) dual-frequency (2.45 and 30 GHz) microwave processing leads to a decrease in the duration of the temperature gradients, and (2) an increase in the heating rate from 5°C/min to 1400°C/min at 2.45 GHz decreases the total required microwave energy by a factor of 55, while at the same time the internal temperature gradients are maintained over a substantially shorter time.     

Recent advances in carbon nanotubes-based microwave absorbing composites

With the blossom of information industry, electromagnetic wave technology shows increasingly potential in many fields. Nevertheless, the trouble caused by electromagnetic waves has also drawn extensive attention. For instance, electromagnetic pollution can threaten information safety in vital fields and the normal function of delicate electronic devices. Consequently, electromagnetic pollution and interference become an urgent issue that needs to be addressed. Carbon nanotubes (CNTs) have become a potential candidate to deal with these problems due to many advantages, such as high dielectric loss, remarkable thermodynamic stability, and low density. With the appearance of climbing demands, however, the carbon nanotubes combining various composites have shown greater prospects than the single CNTs in microwave absorbing materials. In this short review, recent advances in CNTs-based microwave absorbing materials were comprehensively discussed. Typically, we introduced the electromagnetic wave absorption mechanism of CNTs-based microwave absorbing materials and generalized the development of CNTs-based microwave absorbers, including CNTs-based magnetic metal composites, CNTs-based ferrite composites, and CNTs-based polymer composites. Ultimately, the growing trend and bottleneck of CNTs-based composites for microwave absorption were analyzed to provide some available ideas to more scientific workers.     

In situ synthesis and characterization of fluorinated polybenzobisoxazole/silica‐coated magnetic Fe3O4 nanocomposites exhibiting enhanced electromagnetic wave absorption property

A series of magnetic fluorinated polybenzobisoxazole (6FPBO)/silica‐coated Fe₃O₄ magnetic nanoparticles composites were synthesized through in‐situ polymerization. The structure and morphology of the obtained nanocomposites were characterized by X‐ray diffraction, transmission electron microscopy, and scanning electronic microscope (SEM). The SEM images indicated that the crosslinking structure is formed in magnetic 6FPBO nanocomposites. The magnetic and electromagnetic properties of magnetic 6FPBO nanocomposites in the 2–18 GHz frequency range were characterized by vibration sample magnetometer and vector network analyzer, respectively. The calculated reflection loss of the as‐prepared magnetic 6FPBO nanocomposites showed that the best microwave absorption reached to −20 dB at 17 GHz with a matching thickness of 3.00 mm. POLYM. COMPOS., 36:884–891, 2015. © 2014 Society of Plastics Engineers     

Microstructural and microwave shielding characteristics of water‐soluble polypyrrole–polyvinyl alcohol–graphite oxide core–shell nanocomposites

This article reports on a facile route for the preparation of polypyrrole–polyvinyl alcohol–graphite oxide nanocomposites through the polymerization of pyrrole with different concentration (wt%) of graphite oxide using ammonium persulfate as an oxidant. The synthesized nanocomposites were characterized by Fourier transform infrared spectroscopy, and their surface morphologies were studied by scanning electron microscopy and transmission electron microscopy. Their solubility in water, DC conductivity in solution, and the current–voltage characteristics of the nanocomposites were studied. Furthermore, the microwave absorption at 1.0–10.0 MHz and the effects of sample thickness on the microwave absorption were investigated. The composites including higher concentration of graphite oxide showed increased solubility and electrical conductivity, and high electromagnetic shielding effectiveness. POLYM. COMPOS. 2012. © 2012 Society of Plastics Engineers     

Finite element modeling of electromagnetic properties of 3D printed materials in X-band spectrum

Microwave absorbers are distinctive class of materials, which are used to absorb undesired electromagnetic waves in the microwave spectrum. A finite element modeling approach was utilized in the present study to investigate the microwave absorption properties of three different materials such as polycaprolactone (PCL) polymer, polycaprolactone-carbonyl iron particle (CIP) composite with random orientation of CIPs, and polycaprolactone-carbonyl iron particle composite with oriented chain structures of CIPs, fabricated by extrusion-based 3D printing process. A two-dimensional model was formulated comprising the material under test and two rectangular waveguides attached to the vector network analyzer. A two-port network model was used to emulate the real-time experimental environment. It was found from the electric field distribution plot that the oriented PCL-CIP model reflects and transmits marginal amplitude of electromagnetic waves when compared with nonoriented PCL-CIP model followed by pure PCL model. The simulation results were validated with the experimental findings. A minimum loss of electromagnetic energy was observed for oriented model as compared to other models, supporting the experimental findings. Larger domain wall motions and micro capacitor behavior of oriented chains of carbonyl particles were responsible for such microwave absorption properties of oriented composites. The practical utility of the present modeling and simulation approach is multifold as it can be utilized to investigate and compare the electromagnetic shielding behavior of various polymeric composites without any material wastage, machine utilization, and physical testing.     

微波加热用于活性炭的制备、再生和改性

从微波加热的应用着手,阐述了微波辐照的热效应、微波在活性炭的制备、再生和改性过程中的作用机理;指出了微波加热处理活性炭有待进一步研究的问题。     

Controllable synthesis of FeSe2/rGO porous composites towards an excellent electromagnetic wave absorption with broadened bandwidth

Due to the escalating demand for electronic dependability and defense security, there has been a surge in research into broadband and lightweight microwave absorbers. Porous composites that are lightweight and plentiful in interfaces have the potential to be high-performance absorbers due to their ability to attenuate waves in a balanced manner and match impedance. “Using a solvothermal technique we generated FeSe₂/rGO composites with a porous topology. By varying the weight of rGO, the electromagnetic properties of FeSe₂/rGO composites may be finely tuned. Impedance matching and attenuation capability are both improved as a direct result of the porous structure and the appropriate electromagnetic parameters. FeSe₂/rGO composites benefit from the tunable composition, porous structure, and strong synergistic effect between FeSe₂ and rGO sheets and display outstanding microwave absorption performance with an ultrabroad bandwidth approaching 5.2 GHz with a thin thickness of 1.6 mm which covers 75% of the studied frequency range. At the same thickness, a significant reflection loss of ?43.7 dB is attained. This work not only enables the tuning of electromagnetic parameters but also expands the use of high-performance microwave absorption devices. Remarkable microwave absorption ability, of the porous composites FeSe₂/rGO can be utilized as a high-performance microwave absorber.”     

还原的氧化石墨烯/CoFe2O4的膜分散-水热法制备及其吸波性能

基于双膜分散技术与水热法相结合的思想,在较低温度条件下,短时间内合成了还原的氧化石墨烯(rGO)/CoFe₂O₄纳米复合材料,并研究了rGO/CoFe₂O₄的吸波性能。通过 XRD、SEM、EDS、TEM、TG/DSC、IR测试手段对rGO/CoFe₂O₄进行表征,采用矢量网络分析仪测定了复合材料在2~18GHz范围内复介电常数和复磁导率的变化,并利用计算机模拟材料在不同厚度下电磁波的衰减性能。结果表明:在透明绢丝状石墨烯的表面及边缘负载了粒度均匀的纳米CoFe₂O₄粒子;单一纳米CoFe₂O₄的反射率损耗为-3.59dB。而mCoFe₂O₄:mGO为10:7的样品的吸波层厚度在2~3mm之间变化时,微波吸收效果显著增强,厚度为3mm时,出现最大微波衰减值-9.2dB,并且微波吸收峰随着吸波层厚度的增加而向低频移动。相比于单一纳米CoFe₂O₄粉体,rGO/CoFe₂O₄纳米复合材料对电磁波的吸收效果有了大幅度的提高。     

Electromagnetic wave shielding and microwave absorbing properties of hybrid epoxy resin/foliated graphite nanocomposites

The aim of this study is to prepare and characterize foliated graphite nanosheets (FGNs) reinforced composites based on epoxy resin for the electromagnetic wave shielding and microwave absorbing applications. The microstructure of as prepared FGNs and epoxy reinforced with different content of foliated graphite was examined by means of scanning electron microscopy and transmission electron microscopy. The effect of FGNs on thermal stability of composites was examined by thermal gravimetric. It is found that the inclusion of FGNs into the epoxy resin matrix enhances the microstructure core of epoxy resin composites. Static electric properties such as electrical conductivity, carrier mobility, number of charge carriers, and thermoelectric power of composites were studied in details. Dielectric properties of epoxy/FGN composites were characterized as a function of composition and frequency in the range of 1–18 GHz. The electromagnetic wave shielding as a function of frequency of composites was examined and compared with theoretical values. The highest shielding effectiveness was obtained for high foliated graphite loading sample FG40 at frequency of 18 GHz it equals to 62 dB. Finally, the electromagnetic wave properties such as absorption loss and reflection loss as a function of frequency were investigated. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Graphite-like carbon nitride (g-C3N4): A promising microwave absorber

During the last few years, the ever-increasing development of electronic devices using and/or producing electromagnetic waves has exited the global concern related to the electromagnetic pollution. As a result, based on the transmission line theory, widespread microwave absorbing materials have been architected operating according to their permeability and permittivity to mitigate the pollution and their emerged hazards. At frequencies above 1 GHz, dielectric nanostructures, having more specific surface area, gained the considerable attention due to their salient microwave absorbing characteristics, originated from the enhanced dipole, interfacial, and defect polarization, deduced by Debye relaxation and Maxwell-Wagner model. Among them, two-dimensional (2D) nanostructures are under the spotlight owing to their unique electromagnetic features. Interestingly, g-C₃N₄ nanosheets illustrated salient microwave absorbing properties generated from its special conjugated structure synthesized from a decussate arrangement of nitrogen and carbon. The lone pair electrons and sp2 hybridization develop π→π*, n→π*, and n→σ* transitions enhancing interfacial interactions, bringing its outstanding microwave properties. In this study, a comprehensive perspective ascribed to the defect engineering, doping, compositing, and medium, influencing the microwave absorbing properties of g-C₃N₄ have been scrupulously dissected. More significantly, the main origins behind the observed permeability of this type of materials were essentially discussed.     

Development and Application of a Microwave Reactor Radiating through a Leaky Coaxial Antenna

A novel microwave (MW) reactor radiating through a leaky coaxial antenna with an impedance matching unit was developed. The MW energy could be effectively transmitted from the power source to the reactor with a uniform distribution of the electromagnetic field. Simulations of the MW radiating indicated the excellent radiation capacity of electromagnetic waves and a uniform axially extended cylinder MW field around the antenna. The simulation result was in accordance with the antenna efficiency measurements and the temperature distribution. Model reactions of benzoyl hydrazine preparations performed in an open vessel under atmospheric pressure and the preparation of 3‐acetylcoumarin in a sealed reactor under pressure proceeded smoothly on a scale of hundreds of grams.     

Dielectric and microwave properties of carbon nanotubes/carbon black filled natural rubber composites

Abstract

Natural rubber (NR) based nanocomposites containing a constant amount (50 phr) of standard furnace carbon black and carbon nanotube (CNT) at a concentration from 1 to 5 phr have been prepared. Their dielectric (dielectric permittivity and dielectric loss) and microwave properties (coefficients of absorption and reflection of the electromagnetic waves and electromagnetic interference shielding effectiveness) have been investigated in the 1–12 GHz frequency range. The results achieved allow recommending CNTs as second filler for NR based composites to afford specific absorbing properties.     

Decoration of worm-like Cu2S particles on CuCo2S4 micro-spheres: An effective strategy to improve the electromagnetic dissipation feature

Civilization can be shielded from the dangerous electromagnetic spectrum by using microwave absorption materials, however, absorbing electromagnetic radiation with thin thickness and high bandwidth remains a challenge, especially at scales that are significant. Herein, we propose a novel architecture where worm-like Cu₂S particles are decorating CuCo₂S₄ micro-spheres were decorated, and this method is thought to be a successful one for enhancing the created nanocomposite's ability to dissipate electromagnetic radiation. Changing the filler loading percentage allows the nanohybrids' electromagnetic characteristics and microwave dissipation effectiveness to be efficiently changed. This leads to the creation of ultra-bandwidth absorbers with thin thickness, which are then tested using waveguide and free-space techniques. The sample with a thickness of 1.4 mm has a maximum reflection loss of ?18 dB and a maximum bandwidth of 3.6 GHz. The hetero-structures, multi-interfaces, and multiple relaxations phenomena, as well as the combined effects of the two components, are credited with the superior microwave absorption performance compared with the state-of-the-art. This finding demonstrates that CuCo₂S₄/Cu₂S nanohybrids pave the way for the development of future high-performance microwave absorption materials.     

Experimental characterization and modeling of microwave heating of oil palm kernels,mesocarps, and empty fruit bunches

The current work investigates the microwave heating and drying behavior of oil palm fresh fruits and bunches. Dielectric, thermal, and mechanical properties of oil palm kernels (OPKs), mesocarps, and empty fruit bunches (EFBs) were determined. New empirical models of the dielectric constant, loss factor, elastic modulus, and yield stress as functions of moisture content were obtained for OPKs, mesocarps, and EFBs. The thermal conductivity of the mesocarps and EFBs was 0.458 and 0.0285?W/(m K), respectively. Thermal analyses showed that decomposition of OPKs, mesocarps, and EFBs started when the temperature exceeded 100°C. Multiphysics models that consider electromagnetic waves, moisture, and heat conservation as well as material deformation were developed for OPKs, mesocarps, and EFBs. The simulation results show that an EFB sample with high moisture content generated a high moisture gradient during heating, causing high stress that exceeded its yield stress. However, this effect occurred only during the initial heating. Our results show that microwave heating can aid the detachment of oil palm fruit from bunches before EFBs harden because of moisture loss.     

Dielectric Dispersion of Ferroelectric Ceramics and Single Crystals by Sound Generation in Piezoelectric Domains

Periodic domain configurations with alternating 180° and 90° domains are not completely mechanically clamped up to microwave frequencies. Above the acoustic resonance of the ferroelectric sample, therefore, the dielectric constant comprises contributions which can be attributed to the free dielectric constant. Up to microwave frequencies the domains are piezoelectrically active; they emit longitudinal and shear thickness waves into the surroundings which cause dielectric loss in the sample. The dielectric step from the free condition to the clamped condition at the relaxation frequency is on the order of Δε∼ 10–100. This step is much smaller than the step caused by the emission of shear waves from 90° domain walls.     

Reduced leakage current of multiferroic BiFeO3 ceramics with microwave synthesis

Preparation of multiferroic BiFeO₃(BFO) is reported using microwave heating. The prepared sample is characterized using x-ray diffraction, scanning electron microscopy, differential scanning calorimetry and leakage current measurements. It is observed that the BFO can be prepared with microwave heating at a fast heating rate, consisting of more homogeneous microstructure and better electrical properties. Phase purity of the sample is confirmed from x-ray diffraction measurements. Uniform grain size distribution is observed for the sample prepared with microwave heating. More than an order of magnitude reduction in the leakage current is observed for the sample prepared with microwave heating as compared to conventional radiant heating.